As is well known, air flow parameter measurements are extremely useful in controlling the operation of an internal combustion engine. Typical conventional air flow sensors found in many of today's automotive systems operate on the hot wire anemometer principle. Briefly, a hot film or wire is heated by an electrical current so as to maintain a constant temperature differential between the heated element and another non-heated element (i.e., at ambient temperature). The air flowing past the heated element removes heat from the heated element (e.g., with increased air flow removing more heat)--requiring additional electrical heating current to maintain the heated element at the same constant temperature differential above ambient temperature. The electrical current supplied to the heated element may be measured (e.g., by measuring the voltage drop induced across a resistor coupled in series with the heating element or by using the voltage drop in the series resistors to control the current to the elements so as to force the resistance to stay constant) to provide a measure of air flow parameters (e.g., mass air flow).
The following is a by no means exhaustive listing of prior-issued patents relating to this type of air flow sensor:
U.S. Pat. No. 3,623,364 Withrow 30 Nov 1971 PA0 U.S. Pat. No. 4,244,217 Ledbetter 13 Jan 1981 PA0 U.S. Pat. No. 4,501,144 Higashi et al PA0 U.S. Pat. No. 4,135,396 Stanke et al 23 Jan 1979 PA0 U.S. Pat. No. 4,478,076 Bohrer 23 Oct 1984 PA0 U.S. Pat. No. 4,283,944 Gruner et al 18 Aug 1981 PA0 U.S. Pat. No. 4,624,138 Ono et al 25 Nov 1986 PA0 U.S. Pat. No. 4,471,647 Jerman et al 18 Sep 1984 PA0 U.S. Pat. No. 4,637,253 Sekimura et al 20 Jan 1987 PA0 U.S. Pat. No. 3,992,940 Platzer, Jr. 23 Nov 1976 PA0 U.S. Pat. No. 4,733,559 Aine et al 29 Mar 1988 PA0 U.S. Pat. No. 4,672,847 Uchiyama et al 16 Jun 1987 PA0 U.S. Pat. No. 4,548,077 van Putten 22 Oct 1985 PA0 U.S. Pat. No. 4,566,320 Bohrer 28 Jan 1986 PA0 U.S. Pat. No. 4,587,842 Handtmann 13 May 1986 PA0 U.S. Pat. No. 4,596,140 Dorman et al 24 June 1986 PA0 U.S. Pat. No. 4,680,963 Tabata et al 21 Jul 1987
A typical prior art hot-wire anemometer sensing system includes of the following components:
(a) a heated element typically having a positive temperature coefficient of resistance (TCR);
(b) a cold (ambient temperature) element generally made of the same material as the heated element;
(c) a means for comparing the electrical characteristics (e.g., by measuring current flowing through the element) of the heated element with those of the cold element; and
(d) a current supply for supplying current to the heated element so as to maintain a constant temperature difference between the heated element and the cold element.
In this arrangement, the current needed to maintain the constant temperature difference between the two elements is a measure of the fluid flow. As flow increases, more current is needed to heat the hot element since the hot element must supply additional heat to compensate for the heat removed from it by forced convection due to the fluid flow. At the same time, the cold element temperature tracks the temperature of the flowing fluid and thus automatically compensates for changes in the temperature of the fluid.
In the past, fluid flow sensors of the type described above were typically made using thin resistive wires or thick resistive films (or other materials) disposed on a substrate. More recently, however, thin-film technology has been successfully employed to provide such "hot-wire" air flow sensors. The thin-film sensor may be implemented on a silicon substrate using silicon micromachining and thin-film processing techniques. The resulting thin-film sensor is extremely small in size, consumes very little power, is relatively inexpensive to manufacture, and provides excellent sensor characteristics such as rapid response time.
One exemplary simple method for realizing a silicon-based thin-film sensor for sensing fluid flow parameters is to provide a single thin-film heating element formed on a thermally isolated diaphragm window. As described above, the element is self-heated to a certain temperature by supplying a biasing current through it. When the heated element is exposed to fluid flow, it tends to cool down, and its temperature decreases. A feedback circuit connected to the heated element senses and responds to this change (so as to sustain a constant temperature differential between the heated element and an additional element measuring the ambient temperature of the fluid flow). The feedback circuit thus responds by supplying more current to the heated element--with the fluid flow rate being related to this supplemented current.
However, this arrangement has some significant drawbacks. Due to the small size of the sensor, the signal-to-noise ratio (particularly at low flow rates) may be poor, and the sensor is often unduly sensitive to local disturbances. If the chip size is increased to improve this aspect of performance, the cost advantage tends to be degraded. Characteristics of the thin-film materials being used also place limitations on the amount of current that may be flowed through the thin-film elements (and thus upon the operating temperature ranges of the sensor). For example, the properties of some materials deposited in thin-film form change if the sensor is operated at high temperature or current levels. Further, the simple silicon-based sensor has not in the past fully utilized advanced silicon integrated circuit technology that might be used to add on-chip functions such as temperature compensation, linearization, signal conditioning, etc.
Different configurations of silicon-based thin-film mass air flow sensors are described in the above-referenced Lee et al U.S. Pat. No. 4,884,443 (and in the other patent applications referenced above). One version uses two additional sensing resistors to measure the temperature difference between the upstream and downstream fluid flows (providing a heating resistor between the upstream and downstream sensing resistors to heat the sensing resistors). Another version uses a heating resistor and a separate sensing resistor. Briefly, the sensing resistor detects the temperature of the heating resistor, and a control circuit supplies an appropriate current to the heating resistor in response to the resistance change of the sensing resistor so as to keep the temperature of the heating resistor constant.
Although these various thin-film sensors have been successful in their own right, further improvements are possible.
For example, one of the problems with prior "hot-wire" anemometer sensors in the past has been the need to supply a relatively large amount of current to heat the heated element. A "no flow" condition thus typically provides a relatively large signal offset that must be compensated for. This large signal offset limits the dynamic response range of the sensor response to flow (i.e., for a given supply voltage a large part is used to heat the element, leaving a limited voltage above that for the flow signal), and the sensor therefore does not "span" a sufficiently wide range of different fluid flows found in many applications and environment. In addition, high current densities necessary to heat the heated element may actually damage or otherwise alter the characteristics of the element at high flows--this problem becoming particularly acute in thin-film technology (since, for example, as mentioned previously, some materials when deposited in thin film form change their properties if operated at high current levels).
The present invention solves these problems and drawbacks by providing a new and extremely useful fluid flow sensor arrangement having plural heating elements thermally coupled to one another. This arrangement can provide a desirable solution to cope with the limitations caused by single-element thin-film sensors used in the past.
Briefly, one aspect of the present invention is to provide two discrete heating elements closely thermally coupled to one another (e.g., by stacking one element on top of the other). In the preferred embodiment, a constant current is supplied to one of the heating elements (this element may be termed a "transfer heater element"). Preferably, the transfer heating element supplies a substantial portion of the heat needed to maintain the other heating element at a constant temperature above ambient temperature under a particular fluid flow condition (e.g., no flow). The heat generated by the transfer heating element maintains the other heating element (herein termed the "sensor heating element") at some temperature above ambient (and in one particular configuration, substantially at the desired temperature differential above ambient temperature for no fluid flow) due to the close thermal coupling between the transfer heating element and the sensor heating element.
As fluid flow increases, heat is removed from the sensor heating element (and typically also from the transfer heating element) due to forced convection--thereby decreasing the temperature of the sensor heating element below the desired elevated temperature above ambient. The additional heat needed to maintain the sensor heating element above ambient temperature is provided by the sensor heating element itself through self-heating. That is, electrical current is supplied directly to the sensor heating element to cause it to itself generate sufficient additional heat necessary to maintain its own temperature above ambient temperature. The magnitude of this electrical current supplied to the sensor heating element may be measured (indirectly) as an indication of fluid flow parameters (i.e., mass air flow).
Thus, the transfer heating element essentially determines the initial operating temperature of the heating element (this initial operating temperature may be set by a constant current supplied to the transfer heating element). The sensor heating element electrically isolated from but nearly perfectly thermally coupled with the transfer heating element responds to temperature changes caused by air flow changes by decreasing its resistance (due to its positive temperature coefficient). In response to this decreased resistance, the feedback circuit arrangement coupled to the sensor heating element pumps more current into the sensor heating element so as to maintain the sensor heating element at a constant temperature differential above ambient temperature. The additional current pumped into the sensor heating element may be measured as representative of the fluid flow characteristics.
The functional separation of initialization and flow sensing significantly improves low-flow sensitivity, enhances the long term stability of the sensing element, and permits a higher operating temperature. Increased sensor temperature operating range is provided at least in part because most or all of the current flowing through the sensor heating element is attributable to heat removed from the sensor heating element due to increased fluid flow--thus reducing the current density within the sensor heating element (and at the same time reducing the offset in the signal derived from the sensor heating element current flow). There is no penalty in chip size if the transfer and sensor heating elements are vertically stacked, and a higher degree of integration (e.g., building an on-chip full resistor bridge) is possible or facilitated if the transfer and sensor heating elements are made from different materials. Sensor response time can also be improved significantly without substantially increasing power consumption.
The present invention also provides a silicon-based air flow sensor of the type described above fabricated in a new overall process involving micromachining and standard integrated circuit fabrication processes. Preferably, a thin silicon diaphragm window is provided upon which is formed the transfer heating element (e.g., using thin-film technology). The sensor heating element may be formed on top of (or very close to) the transfer heating element and should be electrically insulated from the transfer heating element (e.g., by a dielectric material such as silicon dioxide or silicon nitride disposed between the two elements). The transfer and sensor heating elements may eventually be coated with a passivation layer (e.g., PECVD nitride or oxynitride). The fabrication process may include forming a deep diffusion doped layer around the diaphragm window, multiple dielectric layers for the diaphragm windows, a resistive thin-film layer for the transfer heating element, a dielectric layer on top of the transfer heating element for electrical insulation (if the transfer and sensor heating elements are stacked), etched holes in the insulation layer to provide electrical conductor access, a second resistive thin-film layer for the sensor heating element, an additional passivation layer, etched holes through the passivation for interconnects, and a diaphragm window (e.g., using a silicon anisotropic etching process).
The diaphragm window in this arrangement provides a good thermal insulation for the heating and insulating elements and low thermal mass to allow fast response to temperature change. This dielectric diaphragm also provides good electrical insulation.
The present invention exhibits many advantages compared to prior air flow sensor designs, including the following:
significantly higher sensitivity to low-velocity flow; PA1 more flexibility to configure compensation arrangements (e.g., due to the additional heater element which is outside of the control loop); PA1 less current demand for the sensing element, thus allowing higher operating temperatures and longer lifetime; PA1 more effective use of a given diaphragm area; PA1 improved response rate without proportionally higher power consumption (heat conductance can be significantly increased with a less significant increase in heat capacity); and PA1 better potential to add further resistive elements on the same sensor chip without sacrificing chip area (e.g., by vertical stacking of elements and/or the use of different resistive materials for different elements).